Typically, the coefficient of thermal expansion of a printed circuit board on which a large scale integration (LSI) or an LSI package is mounted is about 17 ppm/degrees C. The coefficient of thermal expansion is determined depending on the material of copper wiring patterned on the substrate (i.e., a base material). Recently, however, there have been needed printed circuit boards of low coefficient of thermal expansion of about 3 to 3.5 ppm/degrees C., which is about the coefficient of thermal expansion of a silicon wafer.
Examples of materials used for such printed circuit boards of low coefficient of thermal expansion include resin materials of low coefficient of thermal expansion, such as bismaleimide triazine resin (BT resin). A prepreg (i.e., “pre-impregnated”) material is used as a base material of the printed circuit board. The prepreg material is prepared through impregnation of a resin material in a fiber material, such as glass. In order to prepare materials of lower coefficient of thermal expansion, not only the resin materials but the fiber materials are suitably selected. For example, instead of using a general E glass fiber (coefficient of thermal expansion: about 5.5 ppm/degrees C.; modulus of elasticity: about 70 GPa), a low coefficient thermal expansion glass fiber such as a T glass fiber (coefficient of thermal expansion: about 3 ppm/degrees C.; modulus of elasticity: about 80 GPa), is used. The suitable selection of the resin material and fiber material achieves a base material of a printed circuit board of low coefficient of thermal expansion. However, even if materials are thus selected, it is still difficult to achieve coefficient of thermal expansion closer to that of a silicon wafer since the coefficient of thermal expansion of the base material of the printed circuit board is about 12 ppm/degrees C. or greater because the coefficient of thermal expansion of the resin material has restriction.
Examples of the fiber materials used as the prepreg include fiber threads (i.e., yarns) which are a bunch of thin glass fibers called filaments. These fiber threads are knitted alternately in horizontal and vertical directions to form fabric. Alternatively, these fiber threads are arranged closely and in parallel on a plane (i.e., not woven, to form nonwoven fabric). The prepreg material is prepared through impregnation of resin in the fabric or nonwoven fabric to form semi-cured resin (B-stage resin). Here, it is proposed, for example, to flatten the fabric under pressure or knit the fabric using extended flat bundles of threads. These techniques are implemented to obtain thin prepreg materials.
The printed circuit board typically has through holes which penetrate the substrate. In the printed circuit board in which a prepreg material is incorporated, the fiber material (i.e., the fiber threads) of the prepreg material is cut at through hole sites. Therefore, a difference in coefficient of thermal expansion exists in the horizontal and vertical directions of the substrate depending on the positions in which the through holes are formed. Such a difference in the coefficient of thermal expansion causes a difference in coefficient of thermal expansion in the horizontal and vertical directions. The property of a substrate with different physical property in the horizontal and vertical directions is called “anisotropy.”
It is therefore needed to reduce anisotropy of a substrate in a printed circuit board in which a prepreg material is incorporated.
Related art is disclosed in Japanese Laid-open Patent Publication No. 2004-289114, Japanese Laid-open Patent Publication No. 2003-324253, Japanese Laid-open Patent Publication No. 2003-142826, and Japanese Laid-open Patent Publication No. 2001-332828.
Related art is also described in the following Nonpatent literatures: Tohru YAMAGISHI et al., “Structural Analysis of Anisotropic Composite Structures by FEM (Part I)”, 1999.; and S. W. Tsai, “Structural Behavior of Composite Materials, NASA CR-71, 1964.”